It would be impossible to make a boring programme about the life of the undercover cop Mark Kennedy, known to his environmental activist "friends" as Mark Stone. It is a classic boys' own adventure, a man set loose from all the responsibilities of life, cut free from his family and excused the tedium of workplace. Kennedy notes near the start of Confessions of an Undercover Cop (Channel 4) that in the seven years he spent incognito with environmental activists (even that timescale has a mythological ring to it that makes it sound grand and racy), the only times he set foot in a police station were when he was arrested.

Kennedy got to the very top of the activists' movement almost immediately. He tacitly suggests his ascent was due to his superior ability to deceive, but I'm not sure I bought that. I think it was more probably because he had a van. (How many environmental activists would have their own van, realistically? The police could probably have saved themselves a lot of bother by anonymously donating them a bugged van.)

At one point, Mark (I'm going to stick with the one constant element of his name) demonstrates how he would answer questions about his past. "Legend-building", they call it: "What football team do I support? Chelsea. True blue, all the way through." He doesn't sound like a Chelsea fan at all; he sounds wooden and annoyed, like an uncle who's been made to play charades.

But it must have been the cameras, throwing him off his game, because it is inarguable that the activists accepted him as one of their own. In what I thought was the film's most poignant moment, Mark describes how he was treated, having been released from hospital after police had stamped on his spine during a demonstration. "The solidarity and the support and the love that I was shown by the people I was infiltrating brings tears to my eyes. That is a huge part of the guilt that I feel, because of the way I was looked after. People showed me so much care, more care than I was ever shown by the police."

But of course, that's the problem – this James Bond character, who can swoosh in and out, his impervious heart untroubled, is only barely credible in Hollywood and can't take five minutes of real life. The story immediately becomes one of awful compromises – the greatest of which is, of course, the fact of Mark's relationship with another activist (the narrator notes drily: "Opinions vary as to how many women Mark Stone slept with").

"I shared a lot with her," he insists of his long-term girlfriend, who discovered his true identity after four years with him when she found his real passport. "I shared my passion and care for her …" Erm … I suppose. It's not exactly sharing, though, is it? That's not what they mean when they tell you to share things in Relate. "The love and care and affection I showed to her, there was no doubt about that at all," he continues. So that's all right then.

Mark clearly thinks of himself as a man more sinned against than sinning, and I don't think anyone would counter that the police behaved well here – there are broad issues: why were they spending millions of pounds tracking protesters, of whom not one was ever prosecuted as a result of nearly a decade's surveillance? There are specific issues: can you, in good conscience, arrest anyone for the presence of bolt cutters near a fence, when your own officer planted them there? And there are issues pertaining to the treatment of Mark and his "girlfriend" (I'm havering with those quote marks – another woman who alleges he slept with her has said she feels "violated" by the deception. So his "girlfriend" may well prefer the term "victim" or "mark"); why didn't they notice he was having a relationship? If they did notice, why didn't they intervene? Why was his debrief handled so badly?

It's a sad, unedifying story, but told in a slightly relativistic way: Mark was its focus but doesn't seem to have achieved much understanding and isn't prepared to take much responsibility. It makes his testimony sound plaintive and his character hard to love.

Andrew Graham-Dixon, by contrast, is incredibly easy to love on Art of America (BBC4). He looks like an idealised court portrait of Alan Partridge – cleverer, more handsome, not a twerp – and he has an evocative, dry, unfussy turn of phrase. Describing the first wave of British settlers arriving in America, unprepared for the clement weather, he asks us to picture "a whole troop of sweaty Elizabethans clanking their way into what is now the forests of Virginia"… and you really can, can't you?